The present invention relates to an actuation device for an electrohydraulic brake system of the xe2x80x98brake-by-wirexe2x80x99 type which includes a first piston (push rod piston) that is operable by means of an actuating pedal and biased by a first resetting spring, and a second piston (floating piston) biased by a second resetting spring, the said pistons being arranged one behind the other in a housing and limiting pressure chambers that are in connection with an unpressurized pressure fluid supply reservoir, the said pressure chambers being closable by means of a first and a second valve device, wherein at least the first resetting spring is designed as a travel simulator spring that determines the pedal characteristics, with a sealing sleeve which is arranged at the second piston and limits the first pressure chamber that is assigned to the first piston, and with a third valve device which closes or opens a hydraulic connection between the first pressure chamber and the pressure fluid supply reservoir.
An actuation device of this type is e.g. disclosed in DE 196 51 153 A1. The special features of the state of the art actuation device relate to the design of the above-mentioned third valve devices which are configured as so-called central valves and, compared to a conventional tandem master brake cylinder, have considerably longer lost travels or closure travels. The extended lost travels permit a movement of the hydraulic pistons during braking by independent force, in the course of which brake fluid cannot be conducted from the master brake cylinder into the wheel brake cylinders because the first and second valve devices are closed. However, the lost travels encountered upon failure of the independent brake force, which are felt at the actuating pedal, are considered disadvantageous in the prior art actuation device.
In view of the above, an object of the present invention is to disclose an actuation device of the type mentioned hereinabove wherein only a short lost travel must be covered in the event of failure of the independent brake force.
A first solution of this object includes that the third valve device is adapted to be closed by the movement of the second piston in relation to the housing, and is formed by the sealing sleeve and a channel that is designed in the area of movement of the sealing sleeve and connected to the pressure fluid supply reservoir. This measure permits achieving a deactivation of the simulator effect during emergency operation, with a view to reaching short pedal travels.
In a particularly favorable aspect of the object of the present invention, a short overall length is achieved because the first piston is guided in the housing and also by means of the second piston. For this purpose, the first piston includes a bore which accommodates a cylindrical extension of the second piston.
In another favorable embodiment of the present invention, the cylindrical extension in the first piston confines a hydraulic chamber which is in connection to the pressure fluid supply reservoir by way of a hydraulic resistor.
The hydraulic resistor is preferably dependent on the position of the actuating pedal. An effective attenuation during actuation of the simulator is achieved by the above-mentioned measures.
To ensure that the pressure which is introduced into the wheel brakes connected to the first pressure chamber can be reduced in a situation where the third valve device is closed and the valve devices that close the pressure chambers are opened, e.g. due to a defect, it is disclosed in the present invention that there is a connection between the hydraulic chamber and the first pressure chamber which is closable by means of a fourth valve device. It is especially favorable that the fourth valve device is closable by a relative movement between the first piston and the extension and is formed by a passage provided in the first piston and a sealing sleeve arranged at the extension.
A second solution of the above-mentioned object can be seen in that the third valve device is designed as a valve which is mechanically operable by a movement of the second piston in relation to the housing.
It has proved favorable to use a travel sensor signal for the detection of a driver""s wish. In another embodiment of the subject matter of the present invention, the travel sensor signal is furnished by a travel measuring system integrated in the housing, and the first piston is designed as a component of the measuring system sensing the actuating travel.
In a travel measuring system which is very reliable in operation, signal generator elements of the travel measuring system are fitted to the cylindrical surface of the first piston, and the system""s signal receiving elements are arranged in the housing.